Efficient removal and recovery of toxic heavy metals from industrial waste streams prior to discharge is a major challenge. Not only does it eliminate acute toxicity of the waste, but also it prevents metal accumulation in biological sludge, which can have severe long term environmental consequences.
Solvent extraction has been a useful operation in separation processes, especially in hydrometallurgical processes. It may be used to remove efficiently toxic heavy metals, e.g., Zn, Cu, Cr, Ni, Cd, and Hg from an effluent to environmentally acceptable levels and recycle these metals to the original processes. In this operation, a liquid solvent S is used to extract a solute (or solutes) M from a second liquid F in which M is dissolved. Solvent liquids S and F are immiscible or substantially immiscible. In conventional operation, solvent liquids S and F are mixed directly, then separated into two phases.
There are some disadvantages inherent in this operation, however. For example, during the direct mixing, an emulsion will be formed that will not only reduce the mass transfer efficiency but result in loss of the extraction solvent S and lower solute M recovery. Conventional solvent extraction also consumes more energy and capital. This technology provides little flexibility with respect to changes in flow rates.
A nondispersive solvent extraction technique has been developed by Sirkar (Sirkar, U.S. Pat. No. 4,789,468 (1988)). Sirkar, U.S. Pat. No. 4,997,569 (1991), describes phase interfaces immobilized at the pore mouths of porous/microporous flat membranes or hollow fiber membranes. For example, an aqueous liquid F flows through the bore or the shell-side of a hollow fiber module. The hollow fibers are microporous or porous, and are usually hydrophobic, in which case the aqueous feed does not wet the pores of hydrophobic fibers. An organic extractant S flows on the other side (shell side or the fiber bore) of the hollow fiber module and wets the fiber pores. If the aqueous solution pressure is equal to or slightly greater than that of the organic extractant phase, the aqueous-organic interface is immobilized on the aqueous side of the membrane and solvent extraction is achieved through this immobilized interface. As long as the aqueous phase pressure does not exceed the organic phase pressure by an amount called the breakthrough pressure (Prasad and Sirkar, Chapter 41 in Membrane Handbook (Ho and Sirkar, Editors), Van Nostrand Reinhold (1992)), the phase interface is immobilized at the pore mouths and nondispersive solvent extraction can be carried out.
To recover the metal extracted into the organic phase, one usually contacts the organic phase with a back-extracting aqueous solution. A thin layer of organic solvent may be used as a membrane to achieve extraction on one surface of the membrane and back extraction at the other surface of the membrane. Organic liquids S immobilized in inert microporous supports can, for example, be used to transfer a solute M between two aqueous solutions F1 and F2. This kind of operation is usually called supported liquid membrane (SLM) separation. The main disadvantage of SLM-s is the lack of long term stability, which probably results from loss of membrane solvent S by solubility, osmotic flow of water across the membrane, progressive wetting of the support pores, and pressure differential across the membrane (Danesi et al., J. Memb. Sci., 31 (2-3), 117 (1987).
A novel but robust liquid membrane structure using hydrophilic or hydrophobic microporous hollow fibers called Hollow Fiber Contained Liquid Membrane (HFCLM) was proposed recently by Sirkar's group (Majumdar et al., AIChE J., 34, 1135 (1988); J. Memb. Sci., 43, 259 (1989); Sengupta et al., AIChE J., 34, 1698 (1988); Guha et al., AIChE J. 40; 1223 (1994)). This HFCLM retains the inherent SLM advantages and overcomes most of its shortcomings. The aqueous-organic interfaces are immobilized in the pores of two sets of commingled highly open microporous polymeric membranes. In this arrangement, the two aqueous phases flow through the porous membrane fiber bore in contact with the liquid membrane present in the shell side between the fiber sets. The membrane pores are filled with the organic phase, preferentially wetting it, while the immiscible aqueous phase is completely excluded. Extraction is easily achieved by transfer of solutes through the aqueous-organic interfaces immobilized at the pore mouths of one set of fibers for feed solution by maintaining a pressure difference between the aqueous feed phase and the stationary liquid membrane phase. Back extraction is achieved similarly at the immobilized organic-aqueous interfaces in the pores of second set of fibers through the bores of which the aqueous "strip" (or back-extraction) solution flows. The basic concept and apparatus are described in two patents by Sirkar (Sirkar, U.S. Pat. No. 4,789,468 (1988); Sirkar, U.S. Pat. No. 4,997,569 (1991)).
The citation of any reference herein should not be deemed an admission that such reference is available as prior art to the invention.